The present invention relates to an apparatus and method for sensing the presence of a magnetic field and particularly to magnetometers and gradiometers for sensing magnetic field gradients.
Magnetometers and gradiometers are well-known devices for measuring and sensing magnetic fields. By way of background, such devices are described in the following articles: IEEE Transactions on Geoscience Electronics, October 1970, "Lunar Surface Magnetometer", by Dyal et al, pgs. 206-208; IEEE Transactions on Magnetics, Vol. Mag-6, No. 2, June 1970, "The Fluxgate Mechanism, Part I: The Gating Curves of Parallel and Orthogonal Fluxgates", by Fritz Prindahl, pgs. 376-382; and IEEE Transactions on Geoscience Electronics, Vol. GE-7, No. 4, October 1969, "A Miniature Two-Axis Fluxgate Magnetometer", by M. Acuna and C. Pellerin, pgs. 252-260.
As described in the above articles, one particular type of vector magnetometer is a fluxgate magnetometer. Many mangnetometers use a toroidal core. A toroidal drive winding on the core is driven by a drive current which switches the core magnetization back and forth between positive and negative saturation. A sense winding also couples the core and detects the magnetic flux that closes outside the core. An electromagnetic force (emf), called a sense signal, is induced in the sense winding as a result of the combination of the drive current and any magnetic field intercepting the sensor. Normally, one magnetic field intercepting the sensor is the earth's magnetic field. Other magnetic fields to be detected may also be present. Whenever a vector magnetometer is moved around in the earth's magnetic field, large variations in the sense signal result as a function of the change in orientation of the sense winding sensitivity axis relative to the direction of the earth's magnetic field. For this reason, vector magnetometers by themselves are not satisfactory for detecting magnetic field anomalies in the presence of the earth's magnetic field unless the magnetometers are stationary in the earth's magnetic field and the anomalies are moved.
In order to overcome the orientation sensitivity of a single vector magnetometer, two magnetometers are mounted some distance apart and are interconnected to form a gradiometer. The two magnetometers are positioned with their sense winding axes antiparallel and with their windings equal in sensitivity. The sense signals from the magnetometers are algebraically added so that the positive sense signal due to the earth's field in one magnetometer tends to cancel the negative sense signal due to the earth's field in the other magnetometer. Under these conditions, changing orientations with respect to the earth's spacially uniform magnetic field tend not to cause any variation in the output signal. Accordingly, gradiometers are well suited for sensing the presence of magnetic field gradients or fields other than the uniform earth's field and in the presence of the earth's field. When gradiometers are battery operated, of light construction, and otherwise suitable for hand-held use, they are frequently employed for locating magnetic objects such as magnetic stakes.
An important construction requirement of such gradiometers is that the two magnetometer sensors have their sense winding axes as nearly antiparallel as possible. To the extent that the sense axes are not antiparallel, unwanted error components are introduced into the output sense signal. The error components, among other things, render the gradiometer sensitive to changes in orientation with respect to the earth's magnetic field.
A number of techniques have been employed to insure that the unwanted error components are minimized or cancelled. In one technique, precision mechanical alignment is employed to balance out the error components. While such an alignment technique may be acceptable in a laboratory environment, it is difficult to maintain proper alignment in portable instruments which are subject to vibration and shock in normal hand-held use. Furthermore, once the alignment is disturbed, realignment is difficult and has not proved entirely satisfactory.
In another prior art technique, a superconducting gradiometer is balanced to eliminate error components using a superconducting disc in the manner described in U.S. Pat. No. 3,976,938. While that solution works well in a superconducting environment, a superconducting gradiometer is not suitable for hand-held, portable operation and, therefore, there still is a need for a solution to the balancing problem.
Accordingly, in light of the above background, it is an object of the present invention to provide an improved gradiometer in which the sensors are readily balanced to remove unwanted error components and remain balanced over long periods of time, even in the presence of vibration, shock and temperature changes.